High electron mobility transistor semiconductor device and fabrication method thereof

ABSTRACT

In a method of forming a semiconductor device on a semiconductor substrate ( 100 ), a photoresist layer ( 102 ) is deposited on the semiconductor substrate; a window ( 106 ) is formed in the photoresist layer ( 102 ) by electron beam lithography; a conformal layer ( 108 ) is deposited on the photoresist layer ( 102 ) and in the window ( 106 ); and substantially all of the conformal layer ( 108 ) is selectively removed from the photoresist layer ( 102 ) and a bottom portion of the window to form dielectric sidewalls ( 110 ) in the window ( 106 ).

TECHNICAL FIELD

The technical field relates generally to semiconductor devicefabrication methods and, more particularly, to semiconductor devicefabrication methods using lithographic techniques.

BACKGROUND

Future imaging and communication systems will extend the need for higherfrequency and bandwidth devices and circuits beyond current devicecapabilities. The current generation of millimeter wave (mmW)transceivers and imagers are operable at frequency bands between 50-120GHz. Further, mmW components, such as amplifiers, operable in G-bandfrequencies (140-220 GHz) have also been developed.

Lithography techniques such as electron beam lithography (EBL) and X-raylithography are conventionally used in semiconductor device fabricationprocesses to manufacture mmW semiconductor devices. In an EBLsemiconductor fabrication process, a semiconductor wafer is first coatedwith a photoresist and a window pattern is developed and made in thephotoresist by an electron beam (e-beam). A metal is deposited in thewindow pattern and over the photoresist. The photoresist is thenstripped or lifted off from the wafer so that only the gate metalremains. The EBL semiconductor fabrication process can achieve a gatehaving a gate capacitance sufficiently low for high frequencyapplications, such as the mmW transceivers discussed briefly above.

The next generation of technologies will operate in the sub-millimeterwave to provide benefits such as higher available bandwidth, reducedradar aperture and instrument size, and narrowed beam widths for radarand remote sensing applications by utilizing frequencies from 300 GHz to3 THz.

However, conventional semiconductor device fabrication processes, suchas the EBL semiconductor device fabrication process discussed above,cannot alone manufacture semiconductor devices such as transistorssuitable for sub-millimeter wave operation. Particularly, the EBLsemiconductor fabrication process cannot achieve a transistor gatehaving a gate capacitance sufficiently low enough for operating infrequencies from 300 GHz to 3 THz.

Further, in addition to imaging and communication systems, there is acontinuing trend in the semiconductor industry toward higher devicedensities. To achieve these high densities there have been, and continueto be, efforts toward scaling down device dimensions at submicron levelson semiconductor wafers. In order to accomplish such high device packingdensity, smaller and smaller feature sizes are required.

It would be desirable to have an EBL semiconductor device fabricationprocess that would enable the manufacture of semiconductor devicessuitable for sub-millimeter wave operation. It would be furtherdesirable for such an EBL semiconductor fabrication process to alsosatisfy the production efficiency and complexity levels of currentsemiconductor device fabrication processes for manufacturing mmWcomponents. It would be further desirable for such an EBL semiconductordevice process to have repeatability and robustness in a manufacturingenvironment.

SUMMARY

The present disclosure concerns a semiconductor device fabricationmethod in which a photoresist layer is formed on a semiconductorsubstrate such as, for example, an indium phosphide (InP) substrate. Awindow is formed in the photoresist layer by electron beam lithography(EBL); a conformal layer comprised of a dielectric material is formed onthe photoresist layer and in the window; and substantially all of theconformal layer is selectively removed from the photoresist layer and abottom portion of the window to form dielectric sidewalls in the window.

The photoresist layer can include a polymethyl methacrylate (PMMA) layeron the semiconductor substrate and a PMMA-MAA copolymer layer on thePMMA layer. The conformal layer can be formed by depositing thedielectric material at a temperature and power at which the PMMA layerand the PMMA-MAA copolymer layer do not flow. The dielectric materialcan include silicon nitride (SiN).

Substantially all of the conformal layer can be selectively removed fromthe photoresist layer and a bottom portion of the window by applying adirectional etch such as an anisotropic inductive coupled plasma (ICP)etch to the conformal layer at a radio frequency (RF) bias power between10-50 W to control plasma energy and at an RF ICP power between 100-500W to control ion generation.

A metal film can be deposited in the window and on the photoresistlayer. The metal film comprises one of titanium, platinum and gold. Thephotoresist layer can then be lifted off to form a metal T-gate from themetal film. The window can be isotropically etched to remove a portionof the conformal layer from the dielectric sidewalls of the window priorto lifting off the photoresist layer.

The present disclosure also concerns a method of forming a metal T-gateon a Group III-V semiconductor substrate, a PMMA layer deposited on thesemiconductor substrate, a PMMA-MAA copolymer layer deposited on thePMMA layer, and a window in the PMMA-MAA copolymer layer and the PMMAlayer formed by EBL. The method comprises depositing a dielectric layeron the PMMA-MAA copolymer layer and over the window; selectivelyremoving the dielectric layer to leave dielectric sidewalls in thewindow; depositing a metal film in the window by evaporation; andlifting off the PMMA layer and the PMMA-MAA copolymer layer to therebyform the metal T-gate from the metal film.

The selectively removing of the dielectric layer to leave dielectricsidewalls in the window can further include applying an ICP etch to thedielectric layer.

Prior to lifting off the PMMA layer and the copolymer layer, thedielectric layer can be partially removed from a portion of the PMMAlayer by an isotropic reactive ion etch. The dielectric layer caninclude SiN.

The present disclosure also concerns a high electron mobility transistor(HEMT) device for a sub-millimeter wave amplifier operating infrequencies from 300 GHz to 3 THz. The HEMT device comprises: a groupIII-V substrate; a first source electrode and a second source electrodedisposed on the substrate; a drain electrode disposed on the substrate;and a first metal gate finger disposed between the first sourceelectrode and the drain electrode, and a second metal gate fingerdisposed between the drain electrode and the source electrode, the firstand second metal gate fingers each comprising a metal T-gate having awidth less than approximately 50 nm and greater than 0 nm.

The substrate can be an indium phosphate substrate with anIn_(0.75)Ga_(0.25)As channel. Each of the first and second metal gatefingers can be the metal T-gate comprising one of titanium, platinum andgold and having a width of approximately 35 nm. The metal T-gateincludes sidewall portions composed of a dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIGS. 1A-1D are diagrams illustrating a semiconductor device fabricationmethod according to various embodiments;

FIGS. 2A-2F are diagrams illustrating a semiconductor device fabricationmethod for fabricating a metal T-gate;

FIGS. 3A-3D are scanning electron micrograph images showing a portion ofthe semiconductor device fabrication method for fabricating the metalT-gate;

FIG. 4A is a scanning electron micrograph image of an exemplary twofinger high electron mobility transistor (HEMT) device fabricatedaccording to the method shown in FIGS. 2A-2F;

FIG. 4B is a scanning electron micrograph image of the metal T-gate ofthe HEMT device of FIG. 4A;

FIGS. 5A-5B are diagrams illustrating RF measurements across 1 to 300GHz on the two finger HEMT device of FIG. 4A;

FIG. 6 is a microphotograph of a 308 GHz single stage amplifierincorporating the HEMT device of FIG. 4A; and

FIG. 7 is a diagram illustrating measured gain for the single stageamplifier of FIG. 6.

DETAILED DESCRIPTION

Various embodiments of a semiconductor device and a fabrication methodthereof will be discussed with reference to the drawings in which likenumbers reference like components, and in which a single referencenumber may be used to identify an exemplary one of multiple likecomponents.

Referring to FIGS. 1A-1D, a semiconductor device fabrication method forforming a semiconductor device will be discussed. A semiconductor devicecan be fabricated on a semiconductor substrate such as, for example, asilicon substrate, or a Group III-V semiconductor substrate, such as,for example, gallium nitride (GaN) or indium phosphide (InP).

Initially, as shown in FIG. 1A, a photoresist layer 102 is formed on thesemiconductor substrate 100. The photoresist layer 102 can be, forexample, a two-layer photoresist film composed of polymethylmethacrylate(PMMA) film and a copolymer film formed on the PMMA film. However, thephotoresist layer 102 is not limited to PMMA, and may be composed of,for example, naphthoquinonediazine/novolak (AZ1450J),polyhexafluorobutylmethacrylate (FBM-120), polybutenesulfone (PBS),polyglycidylmethacrylate (COP) or chlorinated polymethylstyrene (CPMS).

The photoresist layer 102 can be formed on the semiconductor substrate100 by, for example, depositing the PMMA film and copolymer filmsuccessively on the semiconductor substrate 100 while it is spinning ata high speed. However, any photoresist formation process can be used aslong as it results in the semiconductor substrate 100 being evenlycoated with the photoresist layer 102. Subsequently, as those skilled inthe art will appreciate, the photoresist layer 102 can be soft-baked todrive out traces of solvent.

Referring to FIG. 1B, electron beam (e-beam) lithography (EBL) isapplied to the photoresist layer 102 to form a window 106 in thephotoresist layer 102 and to convert the photoresist layer 102 intomesas 104. For example, the e-beam can be applied directly (directwriting) to the photoresist layer 102 to form the window 106.Alternatively, a self-supporting mask can be used with a flood electrongun source which provides a collimated beam of electrons. The mask canthen be imaged directly on the photoresist layer 102 to thereby form thewindow 106. The minimum size of a window that can be achieved by EBL islimited by, for example, the wavelength and light-gathering power of thelens. Direct e-beam writing by EBL alone can achieve a window having aminimum size value of 150 nm. However, this minimum size value is notsufficiently low for semiconductor devices intended to be implementedwithin a sub-millimeter wave component.

Referring to FIG. 1C, conformal deposition of a dielectric material onthe mesas 104 and in the window 106 is performed to form a conformallayer 108 thereon. The dielectric material can include, for example, oneof silicon nitride (SiN), silicon dioxide (SiO₂), or any other materialsuitable as an insulator highly resistant to electric current. Thedielectric material can be deposited by, for example, chemical vapordeposition (CVD), or plasma-enhanced CVD. Also, the conformal layer 108can also be formed by thermal growth of the dielectric material on thesemiconductor substrate 100. For example, if the substrate 100 is asilicon substrate, the conformal layer 108 can be a SiO₂ dielectriclayer grown by epitaxial growth. A SiN dielectric layer can also begrown by exposing silicon to ammonia at temperatures between 1000° C.and 1100° C.

Generally, a variety of deposition or thermal growth techniques may beused to form the conformal layer 108. However, the conformal layer 108should be formed at a temperature and power at which the photoresistfilm of the mesas 104 does not flow. For example, PMMA has a lowsensitivity (3*10⁻⁵ C/cm² for a 20-keV beam) and a tendency to flow atelevated temperatures. Therefore, if the photoresist layer 102 (and thusthe mesas 104) includes PMMA, then the dielectric material of theconformal layer 108 should be deposited at a temperature and power thatis not higher than the maximum non-flow temperature and non-flow powerof PMMA, which can be 90° C.

Referring to FIG. 1D, substantially all of the conformal layer 108 isselectively removed from above the mesas 104 and a bottom portion of thewindow 106 to thereby form dielectric sidewalls 110 in the window 106.As a result, the width of the window 106 shrinks and the width of themesas 104 grows. The conformal layer 108 on the mesas 104 and in thewindow 106 can be selectively etched by, for example, directionaletching. For example, an anisotropic inductive coupled plasma (ICP) etchcan be applied to the conformal layer 108 at a first radio-frequency(RF) bias power to control the plasma energy and at a second RF ICPpower to control ion generation by a plasma system. Plasma systems useRF excitation to ionize a variety of source gases in a vacuum system.The RF power source typically operates at a frequency of 13.56 MHz,which is set aside by the Federal Communications Commission (FCC) forindustrial and scientific purposes. However, the plasma system can alsooperate at frequencies as low as a few hundred kilohertz, and microwaveexcitation is in use in certain systems.

The directional etch should be applied at a power at which thephotoresist layer 102 does not flow. Therefore, if the photoresist layer102 (and thus the mesas 104) includes PMMA, then the first RF bias powercan be, for example, between 10-50 W to control plasma energy and thesecond RF ICP power can be, for example, between 100-500 W to controlion generation.

Thereby, the novel semiconductor device fabrication method shown inFIGS. 1A-1D can form a semiconductor substrate having a window 106 withdielectric sidewalls 110. The dielectric sidewalls 110 can provide awindow 106 having a size that is significantly smaller than the minimumsize value achieved solely by EBL. As a result, the window 106 issufficient for fabricating semiconductor devices intended to beimplemented within a sub-millimeter wave component. Particularly, thewindow 106 can be used in subsequent fabrication steps to form Schoitkycontacts, such as metal T-gates discussed below, having a capacitancesufficiently low for operation in frequencies from 300 GHz to 3 THz.

Referring to FIGS. 2A-2F, a semiconductor device fabrication method forfabricating a metal T-gate will be discussed. The metal T-gate can beused as one of the fingers 404, 408 of the high electron mobilitytransistor (HEMT) device 400 shown in FIG. 4A.

Referring to FIG. 2A, a two-layer photoresist film composed of a PMMAlayer 204 and a PMMA-MAA (MethAcrylic Acid) copolymer layer 206 isdeposited on a Group III-V semiconductor substrate 202. Thesemiconductor substrate 202 is preferably an InP substrate. However,other semiconductor substrates sufficient for forming HEMT devices canbe used. The PMMA layer 204 can include, for example, 4% PMMA and thePMMA-MAA copolymer layer 206 can include, for example, 12% PMMA. ThePMMA layer 204 and the PMMA-MAA copolymer layer 206 have differentdissolution rates during EBL as a result of the different molecularweights. Generally, the dissolution rate increases as molecular weightincreases. Thereby, window formation by EBL results in a window 208having a profile in which the width is comparatively narrow in the PMMAlayer 204 then in the PMMA-MAA copolymer layer 206. That is, the window208 includes a narrow portion in the PMMA layer 204. Further, as shownin FIG. 3A, the width of the narrow portion of the window 208 in thePMMA layer 204 is approximately 108 nm, which is smaller than theminimum size value of 150 nm achieved solely by direct e-beam writing ina photoresist layer composed of a single film.

Referring to FIG. 2B, a dielectric layer 210 is conformally deposited onthe PMMA-MAA copolymer layer 206 and over the window 208. The dielectriclayer 210 can be, for example, SiN, and can be deposited by, forexample, CVD. However, the SiN should be deposited at a temperature anda power that is lower than the temperature and power at which the PMMAlayer 204 and the PMMA-MAA copolymer layer 206 flows. As shown in FIG.3B, the width of the narrow portion of the window 208 in the PMMA layer204 is minimized to approximately 69 nm by the dielectric layer 210.

Referring to FIG. 2C, the dielectric layer 210 is selectively removedfrom the top of the PMMA-MAA copolymer layer 206 and the bottom of thewindow 208 to form dielectric sidewalls 212 in the window 208. Thedielectric layer 210 can be selectively removed by applying adirectional etch such as an anisotropic ICP etch to the dielectric layer210 at a first RF bias power to control the plasma energy and at asecond RF ICP power to control ion generation. However, the directionaletch must be applied at a temperature and power at which the PMMA layer204 and the PMMA-MAA copolymer layer 206 do not flow. The first RF biaspower can be, for example, between 10-50 W to control plasma energy andthe second RF ICP power can be, for example, between 100-500 W tocontrol ion generation. As shown in FIG. 3C, the width of the narrowportion of the window 208 in the PMMA layer 204 including the dielectricsidewalls 212 can be minimized to approximately 29 nm after thedirectional etching.

Referring to FIG. 2D, a metal film 214 is deposited on the PMMA-MAAcopolymer layer 206 and in the window 208 to from a metal T-gate 216.The metal film 214 may be formed by an evaporation process in which ametal such as titanium, platinum and gold or a combination thereof isheated to the point of vaporization and then evaporated to form themetal film 214. The dielectric sidewalls 212 can substantially preventthe PMMA-MAA copolymer layer 206 and the PMMA layer 204 from flowingduring the evaporation of the metal. Successful deposition of the metalfilm 214 in the window 208 is shown in FIG. 3D.

Referring to FIG. 2E, the dielectric sidewalls 212 in the window 208 canbe optionally etched to remove the portion of the dielectric sidewalls212 on a bottom portion 218 of the window 208 in the PMMA-MAA copolymerlayer 206 from a portion 220 of the dielectric sidewalls 212 in the PMMAlayer 204. The etching can be performed by, for example, reactive ionetching (RIE) or isotropic etching.

Referring to FIG. 2F, the PMMA layer 204 and the PMMA-MAA copolymerlayer 206 are lifted off from the semiconductor substrate 202 to therebyform the metal T-gate 216. The PMMA layer 204 and the PMMA-MAA copolymerlayer 206 can be lifted off by, for example, application of a liquidstripper or chemical solvent such as, for example, acetone ormethylethylketone, or by oxidizing the PMMA layer 204 and the PMMA-MAAcopolymer layer 206 in an oxygen plasma system.

Thereby, the novel semiconductor fabrication method shown in FIGS. 2A-2Fcan form the metal T-gate 216 having a width less than 50 nm, which issubstantially smaller than the minimum value provided by EBL alone.Further, as will be discussed more fully below, the metal T-gate 216 hasa gate capacitance sufficiently low enough to permit use in asemiconductor device for a sub-millimeter wave amplifier operating infrequencies from 300 GHz to 3 THz.

Further, evaporation of the metal film 214 and lifting off of the PMMAlayer 204 and the PMMA-MAA copolymer layer 206 result in the metalT-gate 216 having a wedge-like base to the gate, forming a thin pointclose to the top of the gate stem. However, the dielectric sidewalls 220preserve the mechanical stability of the metal T-gate 216 duringhigh-force processes between metallization and passivation of thedielectric layer 210 which can introduce considerable torque on themetal T-gate 216, thereby causing it to bend at the thin point. Further,the dielectric sidewalls 220 eliminate the need for support structuresthat can introduce parasitic capacitances and degrade device performanceand greatly increase the complexity and number of steps in the overallgate fabrication process.

The novel semiconductor device fabrication method shown in FIGS. 2A-2Fhas comparable complexity to conventional 100-150 nm HEMT device gatefabrication processes, thereby suggesting that wafer throughput for themetal T-gate on InP HEMT wafers can be sustained at the historicalproduction level of approximately 100 wafers per week per EBL tool.

The device yield of semiconductor devices manufactured according to thenovel semiconductor fabrication method shown in FIGS. 2A-2F was testedto determine the percentage of test devices that exceeded a device yieldset to a nominal threshold of 1000 mS/mm at 1V for peak transconductance(G_(mp)). Particularly, the device yield over 12 wafers processed inseveral distinct lots was determined. The test device resultsdemonstrated the repeatability and robustness of the process in amanufacturing environment. The average yield for the process wasapproximately 85%, with some wafers yielding as high as 98% of devicestested.

Gate length control and alignment are also key parameters that weretracked for variability. Variation in gate length would cause C_(gs)variations that could limit device performance. Average gate lengths fordevices on each wafer, determined indirectly from CriticalDimension—Scanning Electron Microscopy (CDSEM), were found to be wellcontrolled and for the most part varied only ±3 nm between wafers.

Although the novel semiconductor fabrication method discussed above andshown in FIGS. 2A-2F used a two-layer photoresist film composed of aPMMA layer 204 and a PMMA-MAA copolymer layer 206, it should be notedthat a multilayer photoresist film such as a tri-layer photoresist filmhaving a PMMA-MAA copolymer layer sandwiched between two PMMA layers canalso be used.

Referring to FIG. 4A, a semiconductor device 400 having metal gatefingers manufactured by the semiconductor device fabrication methodshown in FIGS. 2A-2F will be discussed. The semiconductor device 400 isa high electron mobility transistor (HEMT) formed on a Group III-Vsubstrate, preferably an InP substrate with an In_(0.75)Ga_(0.25)Aschannel. The HEMT device 400 is for a sub-millimeter wave amplifieroperating in frequencies from 300 GHz to 3 THz. The HEMT 400 includes afirst source electrode 402, a drain electrode 406 and a second sourceelectrode 410 disposed on the substrate. A first metal gate finger 404is disposed between the first source electrode 402 and the drainelectrode 406, and a second metal gate finger 408 is disposed betweenthe drain electrode 406 and the source electrode 410.

The first and second metal gate fingers 404, 408 are each formed fromthe metal T-gate formation process shown in FIGS. 2A-2F to therebyinclude a metal T-gate having a width less than approximately 50 nm andgreater than 0 nm. Particularly, as shown in FIG. 4B, each of the metalgate fingers has a width of approximately 35 nm. The first and secondmetal gate fingers 404, 408 are composed of a metal such as titanium,platinum, gold or a combination thereof. Further, as shown in, forexample, FIG. 2F, each of the gate fingers 404, 408 can include sidewallportions composed of a dielectric layer such as SiN.

The 35 nm metal T-gate has a gate capacitance (C_(gs)) that issignificantly low in comparison to the gate capacitance of metal T-gatesfabricated by conventional EBL. For example, a measured gate capacitanceof the 35 nm metal T-gate was approximately 0.083 pF. In comparison, themeasured gate capacitance of a 70 nm metal T-gate manufactured byconventional EBL without the benefit of the dielectric sidewalls was0.126 pF. That is, forming a 35 nm metal T-gate according to the novelsemiconductor fabrication method discussed above lead to a 35% reductionin gate capacitance.

Electrical validation of the two finger HEMT device 400 with the 35nmmetal T-gate was determined both through DC and RF measurements. The DCcharacteristics of the device 400 were excellent, with typical G_(mp) at1V operation of 1500 mS/mm. Good device pinchoff and output conductancewere also achieved, with typical measured output conductance of 300mS/mm, pinchoff current less than 5 mA/mm, and gate leakage less than−0.2 mA/mm at 2V gate-drain voltage.

Referring to FIGS. 5A-5B, RF measurements were measured across 1 to 300GHz on the two finger HEMT device 400 with extended coplanar referenceplanes at a drain voltage (V_(d)) of 1V and a drain current (I_(d)) of 9mA. To obtain the broadband performance plots, measurements on threeseparate vector network analyzer (VNA) test sets were performed,including an XF probe station (1-100 GHz), a WR-5 based probe station(140-200 GHz) and a WR-3 based probe station (220-270 GHz). Measurementson three separate devices 400 were combined to form the total responsedue to the different probe footprints and calibration structures thatare required to cover each section of the bandwidth. However, themeasured S-parameter trends and the trends of maximum available gain(MAG) and forward current gain (H21) are well pronounced, supporting thevalidity of the measurement and suggesting good process control.

As shown in FIG. 5A, the RF measurements indicate that the cut-offfrequency f_(T) exceeds 400 GHz. Further, as shown in FIG. 5B,extrapolation of MAG down to unity gain with a −20 dB/decade slopesuggests a maximum frequency f_(max) exceeding 600 GHz. Thesemeasurements demonstrate a 40% improvement in f_(T) achieved by reducinggate length from 70 nm to 35 nm by use of the dielectric sidewalls forreducing the gate width, in good agreement with the measured 35%reduction in C_(gs) observed after decreasing the gate width.

Referring to FIG. 6, a single stage sub-millimeter wave MonolithicIntegrated Circuit (SMMIC) prematched amplifier 600 was designed andfabricated using the 2-finger HEMT device 400 in a common-gateconfiguration. The S-parameters of the amplifier were measured directlyon a 25 mm wafer. Referring to FIG. 7, a 4.4 dB peak gain was measuredat 308 GHz for the amplifier 600, making this, to the knowledge of theinventors of the present disclosure, the highest frequency gainamplifier reported to date and the first ever amplifier gain measured inthe SMMW band>300 GHz.

The apparatuses and methods discussed above and the inventive principlesthereof are intended to and will manufacture a semiconductor devicehaving a gate capacitance sufficient for high frequency applications ina sub-millimeter wave device utilizing frequencies from 300 GHz to 3THz. It is expected that one of ordinary skill given the above describedprinciples, concepts and examples will be able to implement otheralternative procedures and constructions that offer the same benefits.It is anticipated that the claims below cover many such other examples.

1. A method of forming a semiconductor device on a semiconductorsubstrate, the method comprising: forming a photoresist layer on thesemiconductor substrate; forming a window in the photoresist layer;forming a conformal layer comprised of a dielectric material on thephotoresist layer and in the window; and selectively removingsubstantially all of the conformal layer from the photoresist layer anda bottom portion of the window to form dielectric sidewalls in thewindow.
 2. The method of claim 1, further comprising: depositing a metalfilm in the window; isotropically etching the window to remove a portionof the conformal layer from the dielectric sidewalls of the window; andlifting off the photoresist layer to form a metal gate from the metalfilm.
 3. The method of claim 1, wherein the forming of the photoresistlayer further includes: forming a polymethyl methacrylate (PMMA) layeron the semiconductor substrate; and forming a polymethylmethacrylate-methacrylic acid (PMMA-MAA) copolymer layer on the PMMAlayer, wherein the forming of the conformal layer further includesdepositing the dielectric material at a temperature at which the PMMAlayer and the PMMA-MAA copolymer layer do not flow.
 4. The method ofclaim 1, wherein the conformal layer further includes silicon nitride(SiN), wherein the forming of the PMMA-MAA copolymer layer furtherincludes depositing SiN on the photoresist layer at a non-flowingtemperature of the photoresist layer.
 5. The method of claim 1, whereinthe semiconductor substrate comprises an indium phosphide (InP)substrate.
 6. The method of claim 1, wherein the selectively removingsubstantially all of the conformal layer from the photoresist layer anda bottom portion of the window to form dielectric sidewalls furtherincludes applying an anisotropic inductive coupled plasma (ICP) etch tothe conformal layer.
 7. The method of claim 1, wherein the selectivelyremoving substantially all of the conformal layer from the photoresistlayer and a bottom portion of the window to form dielectric sidewallsfurther includes applying an anisotropic inductive coupled plasma (ICP)etch to the conformal layer at a radio frequency (RF) bias power between10-50 W to control plasma energy and at an RF ICP power between 100-500W to control ion generation.
 8. The method of claim 1, wherein theselectively removing substantially all of the conformal layer from thephotoresist layer and a bottom portion of the window to form dielectricsidewalls further includes directionally etching the photoresist layerand the window.
 9. The method of claim 1, further comprising: depositinga metal film in the window and on the photoresist layer; and lifting offthe photoresist layer to form a metal T-gate from the metal film. 10.The method of claim 9, wherein the metal film comprises one of titanium,platinum and gold.
 11. The method of claim 1, wherein the forming of thewindow in the photoresist layer further includes forming the window byelectron beam lithography (EBL).
 12. A method of forming a metal T-gateon a Group III-V semiconductor substrate, a polymethyl methacrylate(PMMA) layer deposited on the semiconductor substrate, a polymethylmethacrylate-methacrylic acid (PMMA-MAA) copolymer layer deposited onthe PMMA layer, and a window formed in the PMMA-MAA copolymer layer andthe PMMA layer, the method comprising: depositing a dielectric layer onthe PMMA-MAA copolymer layer and over the window; selectively removingthe dielectric layer to leave dielectric sidewalls in the window;depositing a metal film in the window by evaporation; and lifting offthe PMMA layer and the PMMA-MAA copolymer layer to thereby form themetal T-gate from the metal film.
 13. The method of claim 12, whereinthe selectively removing of the dielectric layer to leave dielectricsidewalls in the window further includes applying an anisotropicinductive coupled plasma (ICP) etch to the dielectric layer.
 14. Themethod of claim 12, further including prior to lifting off the PMMAlayer and the copolymer layer, partially removing the dielectric layerfrom a portion of the PMMA layer by an isotropic reactive ion etch. 15.The method of claim 12, wherein the dielectric layer comprises siliconnitride (SiN).
 16. A high electron mobility transistor (HEMT) device fora sub-millimeter wave amplifier operating in frequencies from 300 GHz to3 THz, comprising: a group III-V substrate; a first source electrode anda second source electrode disposed on the substrate; a drain electrodedisposed on the substrate; and a first metal gate finger disposedbetween the first source electrode and the drain electrode, and a secondmetal gate finger disposed between the drain electrode and the sourceelectrode, the first and second metal gate fingers each comprising ametal T-gate having a width less than approximately 50 nm and greaterthan 0 nm.
 17. The semiconductor device of claim 16, wherein thesubstrate further comprises an indium phosphate substrate with anIn_(0.75)Ga_(0.25)As channel.
 18. The semiconductor device of claim 16,wherein each of the first and second metal gate fingers comprises themetal T-gate comprising one of titanium, platinum and gold.
 19. Thesemiconductor device of claim 16, wherein each of the first and secondmetal gate fingers comprises the metal T-gate having a width ofapproximately 35 nm.
 20. The semiconductor device of claim 16, whereineach of the first and second metal gate fingers comprises the metalT-gate, wherein the metal T-gate includes sidewall portions composed ofa dielectric material.